Who passes on message sent to prison guard? (9)

I believe the answer is:

forwarder

I'm a little stuck... Click here to teach me more about this clue!

'who passes on message sent' is the definition.
I can't judge whether this defines the answer.

'to prison guard?' is the wordplay.
'to' becomes 'for'.
'prison guard?' becomes 'warder' (I've seen this before).
'for'+'warder'='FORWARDER'

Can you help me to learn more?

I've seen this clue in The Independent.

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